This invention relates to thermostatic control systems and, more particularly, to a thermostatically operated gas control system from controlling the amount of fuel gas supplied to a gas burner associated with natural gas and oil processing equipment.
Equipment used for the production and processing of liquid and gaseous petroleum products often incorporates a burner for heating of the produced fluid and/or some other process fluid. These burners consume fuel gas, most often natural gas available at the site, as the source of heat energy. For example, a dehydrator such as shown in U.S. Pat. Nos. 3,094,574; 3,288,448 and 3,541,763, may be located near the well head of a producing gas well to remove water vapor from the gas before it is introduced into the transmission line. Failure to dehydrate natural gas from wells in freezing weather often results in production lines which are plugged with ice; hence, gas dehydration in many areas, particularly in winter, is a necessity. Some fuel gas must be consumed in the dehydration process and the amount consumed represents a reduction in the quantity available for sale. Any reduction in the quantity of fuel gas required for dehydration or other production processes represents an energy savings.
The present invention provides a means of significantly reducing the fuel gas requirement where burners are employed as a heat source in liquid and gaseous petroleum production, and more generally, where burners may be employed in other processes.
In a typical system used for the heating of some process fluid in oil and gas production equipment, the process fluid is contained, while being heated, in a vessel through which the fluid flows. The heating is accomplished by heat transfer from a fire tube heated from within by the products of combustion of fuel gas (mostly methane) and air. Primary air and fuel gas are combined in the burner mixer and discharged through the burner tip. This initial fuel-air mixture is then combined with additional air and burned in the combustion zone of the fire tube. The burner is mounted within a burner housing with the tip protruding into the fire tube. Air enters the burner housing through a flash arrestor. The products of combustion exit the system through a stack. The differential pressure necessary to draw air through the flash arrestor into the burner housing plus overcome friction loss in the fire tube and stack is provided by the combined effect of the stack draft and the momentum increase of the gases in the burning zone.
Control of the temperature of the process fluid is achieved with a thermostat and motor valve which regulate the pressure of the fuel gas supplied to the burner. Two types of thermostat/motor valve actions may be employed. One is a "snap" action wherein the burner gas pressure is either P.sub.s (full regulated supply pressure) or zero (fully off). With this action, a small increase in the process fluid temperature above the set point temperature results in closure of the motor valve, hence zero burner supply pressure. Alternately, a small decrease in process fluid temperature below the set point results in full opening of the motor valve, hence full supply pressure to the burner. The burner is, therefore, alternately fully on or fully off in maintaining a nominal set temperature.
A second type of thermostat/motor valve action is termed "throttling" and results in a burner supply pressure which is continually regulated to hold the set temperature. With this action, the burner supply pressure generally holds approximately constant with time unless the heat load of the system changes because of a change in atmospheric conditions or a change in the flow rate of the process fluid. The throttling type thermostat/motor valve action is generally preferred since it is more efficient and saves energy. A throttling thermostat/motor valve action, when used in gas dehydration, can result in an indirect but particularly significant gas savings. This savings occurs because first with a throttled burner, all or a portion of the gas consumed by the glycol pump employed on the dehydrator can be directed to the burner as fuel gas. In the past no satisfactory thermostat has been available to achieve the control required to maintain a throttled burner action on, for example, a natural gas dehydration unit. In addition, with a snap acting thermostat, the burner is fired at a much higher heating rate than is required to maintain the process temperature resulting in higher stack temperatures and heat energy losses, and during the off cycle of the burner, cold air is drafted through the fire tube creating additional energy losses.
The present invention is a system for controlling the amount of natural supply gas delivered from a natural gas supply source to a natural gas burner adapted to continuously heat a process fluid and for enabling the gas burner to continuously automatically maintain the temperature of the process fluid within a predetermined minimum range of temperatures above and below a pre-set nominal temperature. The system comprises a gas operated regulator means associated with a main supply gas line connected to the gas burner for regulating the amount of supply gas delivered to the gas burner in accordance with the pressure of control gas delivered to the regulator means from a control gas line; a thermostatically operated valve means associated with the control gas line for controlling the pressure of control gas to the gas operated regulator means including a first small fixed size orifice and a second variable size orifice which provide control gas venting means for reducing and increasing the pressure of the control gas in the gas operated regulator means; and temperature responsive linearly movable control means operable in response to changes in the process fluid temperature and being operably associated with the control gas venting means for continuously variably controlling the pressure of control gas in the valve means in accordance with the temperature of the process fluid whereby the pressure of supply gas delivered to the burner is increased when process fluid temperature falls below the set nominal temperature and is decreased when process fluid temperature rises above the set nominal temperature.